Solar panels are by far the most expensive item in a solar panel installation. Understanding the features that differentiate a good solar panel from a bad one is not so straightforward. In several instalments I’d like to give a guide to each of the key criteria to look out for. I will try keep it as simple as possible but it is something that many people ask me about so I think it isn’t a bad idea to discuss these issues in some depth.
First of all I’d like to discuss solar panel efficiency. This defines how effective a solar panel is in converting sunlight into electricity for a given surface area. The advantage of having a higher efficiency solar panel is that you can get more power out of a small available area. For this reason, high efficiency solar panels are normally priced at a premium and targeted at the domestic market where space is most constrained. High efficiency does not necessarily mean better quality or reliability however – these issues are covered later. Nor does higher efficiency mean better value; in many cases lower efficiency panels are used because they are more cost-effective in places where space utilisation is not so critical.
First of all, how do you find out the efficiency of a solar panel? It’s easy to find out this out for yourself. Remember that the power of a solar panel is given by the power you get out under ‘standard test conditions.’ This means the output is measure when the panel is exposed to a very bright light with an intensity of 1000 Watts per square meter (1000W/m2) at a temperature of 25oC. This is normally expressed in Watts (e.g. 185W or 230W etc) and is the power you will get when the sun is very strong. You can then multiply the module length and width (which is shown on the datasheet) to get the module area. By taking the module power in Watts and the standard test conditions of 1000W/m2 you can determine the module efficiency as follows;
Efficiency = power out / power in = module power / (width x length x 1000W/m2)
When evaluating solar panel efficiency its important to be aware that each solar cell has an efficiency higher than that of the whole solar panel (or module) due to empty space. Therefore make sure to find out which value you are looking at.
In general solar panels you will come across in the UK will be made of silicon (I have discussed thin film panels previously) so the discussion here will focus on these. The highest efficiency silicon solar panels on the market today are between 17% and 18% efficient. The efficiency of silicon solar panels is increasing due to R&D, but improvements are incremental and slow because there are a number of fundamental limitations to the efficiency of silicon solar cells which mean that any drastic improvements in the near future are unlikely. Perhaps I will describe those limitations in another article.
The main factor you will come across that affects module efficiency is whether the module is mono or multi-crystalline. In English this means that the solar cells can easy be made from mono or multi-crystalline silicon. Mono crystalline solar cells consist of a slice of a single, very pure silicon crystal and hence are very efficient due to few defects. Multi-crystalline solar cells, which comprise multiple crystals, are around 1-2% less efficient but are generally more cost-effective to produce. Personally I think it generally makes sense to use mono-crystalline cells for domestic installations where space is at a premium and multi-crystalline cells for larger installations.
Another factor that can affect efficiency is anti-reflective coatings. These are becoming more and more common. Nearly all solar cells have texturing directly on top of them that reduces reflection and now many solar panels come with anti-reflective glass. This generally consists of a textured glass that can be seen as a speckled pattern if you look closely. The improvement of anti-reflective coatings is hard to determine, although some manufacturers claim energy yield enhancements of over 5 percent.
When installing a solar panel system your ultimate goal should always be to get the best return on your investment, which means getting the most power for the lowest price without risking reliability and is dependent on many factors besides efficiency. Whilst there are a number of other technologies on the horizon that can be used to improve efficiency by small amounts, nothing will create a drastic change overnight. Prices of solar panels will continue to fall rapidly as production volume increases (in the same way as many other technology products such as computer memory) but these price falls will be matched by reductions in the feed-in tariff. Therefore don’t worry that installing today’s technology risks being superseded by a miracle solar panel tomorrow. Working in the industry gives you pretty good insight as to what is coming down the line.